RADIATION 73 



the former, as an absorber, is simply due to the fact that 

 the periods of the hot and cold carbonic acid are identi- 

 cal, and that the waves from the flame freely transfer their 

 motion to the molecules which synchronize with them. 

 Thus it is that the tenth ot an atmosphere of carbonic 

 acid, enclosed in a tube four feet long, absorbs 60 per 

 cent of the radiation from a carbonic oxide flame, while 

 one-thirtieth of an atmosphere absorbs 48 per cent of the 

 heat from the same source. 



In fact, the presence of the minutest quantity of car- 

 bonic acid may be detected by its action on the rays from 

 the carbonic oxide flame. Carrying, for example, the dried 

 human breath into a tube four feet long, the absorption 

 there effected by the carbonic acid of the breath amounts 

 to 50 per cent of the entire radiation. Eadiant heat may 

 indeed be employed as a means of determining practically 

 the amount of carbonic acid expired from the lungs. My 

 late assistant, Mr. Barrett, while -under my direction, made 

 this determination. The absorption produced by the breath 

 freed from its moisture, but retaining its carbonic acid, was 

 first determined. Carbonic acid, artificially prepared, was 

 then mixed with dry air in such proportions that the ac- 

 tion of the mixture upon the rays of heat was the same as 

 that of the dried breath. The percentage of the former 

 being known, immediately gave that of the latter. The 

 same breath, analyzed chemically by Dr. Frankland, and 

 physically by Mr. Barrett, gave the following results: 



Percentage of Carbonic Acid in the Human Breath 



Chemical analysis PI ysica'. analysis 



4-66 ... .... 4-66 



5-33 6-22 



SOIBNOB A 



